Based on an idea taken from The Walking Dead comic.
Roamers are groups of Zombies that wander around in packs, always on the move.
Roamers may only enter play once the maximum # of Zombies has been reached. To test to see whether any Roamers arrive roll 1D6. On a roll of 6 the test is successful. Next roll 1D6 again to see how many Roamers arrive. Any group of Roamers will begin play grouped on any 'one' random board edge, determined by rolling 1D4. They must be placed adjacently to each other i.e. touching. In the unlikely event that no squares are available place only the amount allowable.
Once in play, these Zombies are treated in exactly the same manner as normal Zombies.
Roamers do not count towards the maximum # of Zombies allowed.